Memory persistence is critically influenced by retrieval. In rats, a single presentation of a conditioned fear stimulus induces memory reconsolidation and fear memory persistence, while repeated fear cue presentations result in loss of fear through extinction. These two opposite behavioral outcomes are operationally linked by the number of cue presentations at memory retrieval. However, the behavioral properties and mechanistic determinants of the transition have not yet been explored; in particular, whether reconsolidation and extinction processes coexist or are mutually exclusive, depending on the exposure to non-reinforced retrieval events. We characterized both behaviorally and molecularly the transition from reconsolidation to extinction of conditioned fear and showed that an increase in calcineurin (CaN) in the basolateral amygdala (BLA) supports the shift from fear maintenance to fear inhibition. Gradually increasing the extent of retrieval induces a gradual decrease in freezing responses to the conditioned stimulus and a gradual increase in amygdala CaN level. This newly synthesized CaN is required for the extinction, but not the reconsolidation, of conditioned fear. During the transition from reconsolidation to extinction, we have revealed an insensitive state of the fear memory where NMDA-type glutamate receptor agonist and antagonist drugs are unable either to modulate CaN levels in the BLA or alter the reconsolidation or extinction processes. Together, our data indicate both that reconsolidation and extinction are mutually exclusive processes and also reveal the presence of a transitional, or "limbo," state of the original memory between these two alternative outcomes of fear memory retrieval, when neither process is engaged.

Signaling at NMDA receptors (NMDARs) is known to be important for memory reconsolidation, but while most studies show that NMDAR antagonists prevent memory restabilization and produce amnesia, others have shown that GluN2B-selective NMDAR antagonists prevent memory destabilization, protecting the memory. These apparently paradoxical, conflicting data provide an opportunity to define more precisely the requirement for different NMDAR subtypes in the mechanisms underlying memory reconsolidation and to further understand the contribution of glutamatergic signaling to this process. Here, using rats with fully consolidated pavlovian auditory fear memories, we demonstrate a double dissociation in the requirement for GluN2B-containing and GluN2A-containing NMDARs within the basolateral amygdala in the memory destabilization and restabilization processes, respectively. We further show a double dissociation in the mechanisms underlying memory retrieval and memory destabilization, since AMPAR antagonism prevented memory retrieval while still allowing the destabilization process to occur. These data demonstrate that glutamatergic signaling mechanisms within the basolateral amygdala differentially and dissociably mediate the retrieval, destabilization, and restabilization of previously consolidated fear memories.

BackgroundPathological forms of impulsivity are manifest in a number of psychiatric disorders listed in DSM-5, including attention-deficit/hyperactivity disorder and substance use disorder. However, the molecular and cellular substrates of impulsivity are poorly understood. Here, we investigated a specific form of motor impulsivity in rats, namely premature responding, on a five-choice serial reaction time task.MethodsWe used in vivo voxel-based magnetic resonance imaging and ex vivo Western blot analyses to investigate putative structural, neuronal, and glial protein markers in low-impulsive (LI) and high-impulsive rats. We also investigated whether messenger RNA interference targeting glutamate decarboxylase 65/67 (GAD65/67) gene expression in the nucleus accumbens core (NAcbC) is sufficient to increase impulsivity in LI rats.ResultsWe identified structural and molecular abnormalities in the NAcbC associated with motor impulsivity in rats. We report a reduction in gray matter density in the left NAcbC of high-impulsive rats, with corresponding reductions in this region of glutamate decarboxylase (GAD65/67) and markers of dendritic spines and microtubules. We further demonstrate that the experimental reduction of de novo of GAD65/67 expression bilaterally in the NAcbC is sufficient to increase impulsivity in LI rats.ConclusionsThese results reveal a novel mechanism of impulsivity in rats involving gamma aminobutyric acidergic and structural abnormalities in the NAcbC with potential relevance to the etiology and treatment of attention-deficit/hyperactivity disorder and related disorders.

Several studies support that stored memories undergo a new period of consolidation after retrieval. It is not known whether this process, termed reconsolidation, requires the same transcriptional mechanisms involved in consolidation. Increasing evidence supports the participation of the transcription factor NF-B in memory. This was initially demonstrated in the crab Chasmagnathus model of associative contextual memory, in which re-exposure to the training context induces a well characterized reconsolidation process. Here we studied the role of NF-B in reconsolidation. NF-B was specifically activated in trained animals re-exposed to the training context but not to a different context. NF-B was not activated when animals were re-exposed to the context after a weak training protocol insufficient to induce long-term memory. A specific inhibitor of the NF-B pathway, sulfasalazine, impaired reconsolidation when administered 20 min before re-exposure to the training context but was not effective when a different context was used. These findings indicate for the first time that NF-B is activated specifically by retrieval and that this activation is required for memory reconsolidation, supporting the view that this molecular mechanism is required in both consolidation and reconsolidation.